This invention relates generally to steam turbine generator systems, and more particularly to methods for estimating performance of steam turbines using relatively inexpensive sensors.
Large steam turbine-generator systems represent major capital investments for their owners and their economic benefit to their owners varies with the thermal efficiency with which the steam turbines are operated. Owners of large steam turbine generators are vitally interested in maintaining the operating parameters of the system as close as possible to the optimum set of operating parameters as designed for the system and/or developed during operational testing following initial installation of the system. In addition, degradation in performance over time can occur due to deterioration of internal parts and other causes.
As part of the installation procedure of a steam turbine-generator subsystem, it is customary for the owners and/or the contractor or turbine manufacturer to conduct very accurate tests with a variety of special purpose, precision sensors to demonstrate or determine the heat rate of the system. Heat rate is a measure of thermal efficiency of a steam turbine-generator system defined as the number of units of thermal input per unit of electrical power output. One standard test of heat rate is known as the ASME test and is defined in an ASME publication ANSI/ASME PTC6—1976 Steam Turbines. A requirement and characteristic of both of the above tests is accurate instrumentation for temperatures, pressures and flows within a steam turbine along with the resulting generator power output to determine accurately the energy content of such conditions and the resulting power output. The accuracy of measurement is sufficiently great that relatively small measurement tolerances need be applied to the results. Such tests are costly to perform. For example, the standard ASME test requires a substantial installation of specialized measuring equipment at a substantial cost in conjunction with a great amount of manpower to administer the test. Besides their cost, ASME-type tests have the additional drawback that they are not suitable for use in day-to-day operation of a steam turbine-generator system.
Because it is expensive to repeat the tests at additional times throughout the lifetime of the turbine, in at least one known method of performance testing of steam turbines, the turbines are provided with a number of “station” sensors to estimate performance.
For example, U.S. Pat. No. 4,891,948 issued Jan. 9, 1990 to Kure-Jensen et al. describes a thermal performance monitor that informs the operator and results engineer of the economic losses, efficiencies, deviations in heat rates and power losses of operating a steam turbine-generator system at its controllably selected pressure and temperature. Specific temperature and pressure signals are generated at various points in the system along with the control valve position signal and the electrical output signal from the electric generator. This data is processed along with the corresponding design values and the economic losses due to temperature deviation, pressure deviation and exhaust pressure deviation from design are calculated. Other calculations produce a comparison of efficiencies of the turbines in the system and consequential power losses.
U.S. Pat. No. 5,327,772 issued Jul. 12, 1994 to Fredricks describes a method and apparatus for determining steam quality wherein heat is added to or removed from a sample flow of steam to reach a point of superheating or supercooling. The amount of energy required to superheat or subcool the sample is factored in with other parameters such as steam flow rate, temperature and pressure to determine the quality of the steam. The steam quality sensor is said to have application in equipment such as turbines, etc.
U.S. Pat. No. 5,621,654 issued Apr. 15, 1997 to Cohen et al. relates to methods and systems for economically dispatching electrical power. Real-time heat rates for a plurality of power generating units, for example, steam turbines, and corresponding emission data for each unit are used to dispatch electrical power at a reduced cost. The method described therein also compares the cost associated with generating power to the cost to purchase power from other electric utilities to achieve savings associated with the dispatching of the electrical power. Each power generating unit includes sensors that are connected to the boiler, steam turbine, and generator. The sensors are known in the art and are provided to measure, for example, water and air temperature and pressure, fuel flow, electrical power, and like characteristics of the power generation portion of the power generating unit. Data generated by each sensor is transferred to a data acquisition interface to be utilized by a plant processor to calculate real time heat rate and to generate a heat rate curve used by the system operator economically dispatch electrical power.
U.S. Pat. No. 5,832,421 issued Nov. 3, 1998 pertains to a method for blade temp estimation in a steam turbine. The method utilizes measurement values including pressure and temperature at locations other than directly at the blades, principally at the input and output stages. Initially, blade temperature is simulated by using a water/steam cycle analysis program as well as by directed experiments. It is also disclosed that in some large steam turbines, temperature measuring devices are installed at respective stages of the HP and LP casings. These measurements provide an indication to the operator or supervising engineer in charge whenever the blade temperature exceeds its limit.
Although station sensors provide useful estimates of performance, station sensors are substantially less accurate than their precision counterparts. Therefore, performance estimates produced using data from station sensors is also less accurate than performance estimates produced using precision sensors.